A Schmitt trigger is a circuit for controlling hysteresis on a digital input of digital circuitry. Within a Schmitt trigger, when the input value is higher than a certain chosen threshold, the output of the Schmitt trigger goes high. Similarly, when the input is below a second lower threshold level, the output goes to a logical “low” level. When the input remains between the high and low threshold levels, the output retains its current value. A Schmitt trigger provides greater stability than a circuit input including only a single input threshold level. With single input threshold level circuits, a noisy input signal near the threshold level could cause the output to switch rapidly back and forth from noise within the signal alone. A noisy Schmitt trigger input signal near one threshold can only cause one switch in output value after which it would have to move beyond the other threshold in order to cause another transition.
Various types of digital circuits such as isolators or isolation drivers demand fast Schmitt triggers with low pulse width distortion (PWD), short delay, low power dissipation and tightly controlled hysteresis transition levels. Traditional Schmitt triggers cannot meet all of these requirements at the same time. Traditional Schmitt triggers are either slow, introduce relatively large PWD, do not have a well controlled hysteresis level or consume relatively high power. Thus, there is a need for a Schmitt trigger that overcomes these problems within a single design and can be used in applications that demand low power, high speed, small PWD and accurately defined hysteresis transition levels.